Bone screws are provided in a variety of shapes and sizes. The most common employ titanium or stainless steel as a preferred material for implantation. These screws are ideal in that they do not oxidize and are very compatible with the bone structure. These screws typically employ one or more helical threads. Many are self-tapping at the proximal tip or end. The distal end of the screw has a head or some physical construction to allow torquing the screw into the bone. The shank extends between the head and the tip end and is machined with the desired threads to engage the bone structure.
The bone structure has an interior of a softer more open trabecular spacing. This region. This region is somewhat spongy or elastic in nature. The outer region of the bone is hard and more densely formed cortical bone. This hard bone can provide an excellent region to cut threads into as the screw is tightened. Historically, the screws, once fully inserted, are used to anchor bone plates, spinal fusion spacers and rods commonly used in spinal surgical repairs and to hold bone fracture fragments together.
The very nature of the helix angle cut into the shank to form the threads creates a spiral ramp inclined slightly. The bone screws are made with the threads being sharp and highly polished to make the bone entry easier. This contributes to a phenomenon that induces the bone screw to loosen over time. Ideally, new bone ingrowth occurs to help hold the fastener in place, but the occurrence of screw loosening is such that many, if not most, bone screws use anti-back out or locking features to keep the bone screw held in the bone in the event the screw tends to loosen. Secondarily, when the screw loosens it can also lower the pull out forces required to tear through the threads in the bone.
These and other issues can be reduced by a recent discovery that was initially created to enhance bone ingrowth on the exterior surfaces of implants. It was discovered that polymers like those used in implantation and metals like titanium and stainless steel could have greatly enhanced bone formation by surface patterning.
In U.S. Pat. Nos. 8,535,388 B2; 8,414,654 B2 and U.S. Pat. No. 8,679,189 B1*, a unique surface pattern was disclosed what in a preferred embodiment mimics under-modelled marine mammal bone. This pattern has proven beneficial in plastic and metal implants including PEEK, stainless steel and titanium. Its continuous network of voids creates regions of enhanced osteoinductivity to enhance rapid new bone growth as explained in U.S. Pat. No. 8,414,654 B2. In practice, the creation of these patterns can be accomplished employing laser technology. This manufacturing ability allows the machining to occur on difficult to reach surfaces and angles. The inventor, when applying this capacity to implants made in the form of bone screws, has found remarkable added benefits which are described hereinafter.